1. Field of the Invention
This invention is generally related to a process allowing an integrated retailer of ophthalmic lenses to furnish a prescription lens with an antireflective (AR) coating on both sides. This invention is particularly related to a process to furnish such an AR coated lens in a short period of time, by applying a thermally curable hard coating and AR coating in a single spin coater.
2. Description of the Related Art
Plastic ophthalmic lenses made from materials, such as polycarbonate and Allyl Diglycol Carbonate (CR-39®, became popular due to their low cost and light weight over glass lenses. Polycarbonate lenses have superior impact resistance compared to CR-39® lenses, and are preferred for applications that require additional safety features. The use of polycarbonate lenses, particularly in the United States, is widespread.
Like glass lenses, plastic lenses suffer from reflective light losses at the air/lens interface. The loss is about seven percent of the transmitted light. The reflective loss is even more severe for lenses made from polycarbonate due to its high refractive index. An effective way to increase the light transmission is to apply an AR coating on the surfaces. AR coatings reduce the reflectance of light from a surface thereby increasing the light transmittance through the coating/substrate interface.
An AR coating, when applied to both sides of a lens, not only increase the light transmission but also improves the cosmetic appearance of the glasses by reducing both internal and external reflections. Night vision is also improved by reducing glare and halos from light sources such as headlights. The reduction of glare further reduces eyestrain when looking at a computer screen or working under fluorescent light for an extend time.
The demand for AR coated ophthalmic lenses has increased and keeps growing due to the aforementioned advantages. Current AR coatings are primarily applied by vacuum deposition techniques such as sputtering or evaporation. Because these methods require lengthy work time, expense, and highly technical equipment, their operations are limited to lens manufacturers, large lens processing labs, or special optical coating facilities. While this practice satisfies the need of AR-coated finished ophthalmic lenses, it fails the market demand for rapid delivery of AR-coated prescription lenses made from semi-finished lens blanks.
In addition to vacuum deposition techniques, AR coatings can also be formed by sol-gel techniques. In a typical sol-gel process, AR coating precursors are made into colloidal solutions and then applied to the substrate through methods such as dip-coating or spin-coating. The coated layer is then thermally cured to a continuous film to provide AR function. An AR coating may contain a single layer or multiple layers. AR coatings from sol-gel techniques are describe in U.S. Pat. Nos. 4,966,812, 5,268,196, 5,476,717, 5,580,819, 5,858,526, and 6,372,354, all of which are herein incorporated by reference. In particular, U.S. Pat. No. 6,372,354 discloses a sol-gel composition and process to form an AR coating on plastic substrates with a curing time of thirty minutes or less.
Sol-gel techniques provide a rapid process to apply AR coatings. The process requires considerably less time, cost, and expertise than vacuum deposition methods. Thus, it enables an integrated retailer of ophthalmic lenses or a small scale optical lab to make a pair of AR coated, prescription lenses from semi-finished lens blanks in a short period of time (e.g., one hour or less).
However, poor adhesion may exist between an AR coating and the lens surface. Consequently, lenses (especially polycarbonate lenses) need an adhesion promoting layer between the AR coating and the lens surface. Hard-coating layers may be the promoting layer. In general thermally curable coating requires long curing time such as several hours. The inventors have found thermally curable hard coating (also known as a Tegra® coating by Vision-Ease) provide good adhesion and also can be cured within a short time by using the process described in the commonly assigned co-pending U.S. Application Ser. No. 10/632,627 filed Jul. 31, 2003 entitled Spin Application of Thermally Cured Hardcoats, the entire contents of which is hereby incorporated by reference.